This invention is directed to a selective hydrogenation of dienes to monoolefins, particularly of cyclopentadiene to cyclopentene. More specifically, it is directed to a process whereby cyclopentadiene is selectively hydrogenated to cyclopentene with the use of a highly dispersed form of nickel as a catalyst and water as a reaction medium.
At the present time, substantial amounts of cyclopentadiene, usually as dicyclopentadiene, are available as a byproduct from the steam cracking of gas oil or naphtha, a process which produces primarily ethylene. Cyclopentene has been found to be useful as a monomer for the formation of general purpose elastomers by ring opening polymerization of cyclopentene. Therefore, it is desirable to convert a portion of the excess cyclopentadiene available into a more valuable raw material, such as cyclopentene.
The hydrogenation of cyclopentadiene to cyclopentene is not new. For instance, in U.S. Pat. No. 2,360,555, issued Oct. 17, 1944, there is disclosed a selective hydrogenation of one of the two conjugated double bonds of a cyclic diolefin to produce the corresponding cyclic monoolefin, which is accomplished by conducting the hydrogenation in the liquid phase in the presence of an active hydrogenation catalyst, under moderate hydrogen pressure, such as 2 to 5 atmospheres absolute, and at relatively low temperatures, such as from 0.degree. to 40.degree. C. and even up to 100.degree. C., using substantially less than the stoichiometric amount of hydrogen theoretically required to completely reduce the cyclic diene to the corresponding cyclic monoolefin. The catalyst therein disclosed is a pyrophoric nickel metal catalyst, such as Raney nickel. It is also disclosed that it is desired to conduct the reaction in dilute solution. The dilution may be affected by the addition of any solvent, stable under conditions of the process and which is not a catalyst poison and whose boiling point is such as to render it easily separable from the reaction mixture. Benzene and ethanol as well as tetralin, dioxane, isooctane, ethyl ether and diisopropyl ether are disclosed as such solvents in such process.
In U.S. Pat. No. 3,819,734, issued July 25, 1974, there is disclosed the hydrogenation of cyclopentadiene to cyclopentene by bringing cyclopentadiene into contact with a catalyst consisting essentially of (1) nickel, on a magnesium or zinc oxalate support, (2) a ligand selected from the group consisting of trimethyl phosphine, triethyl phosphine, methyl ethyl propyl phosphine, trimethyl phosphite, triethyl phosphite, tributyl phosphite, triphenyl phosphite, etc., while in the presence of hydrogen, at temperatures from 0.degree. C. and at pressures from 0 to 1000 pounds per square inch gauge. The solvent mentioned therein is ethanol.
In U.S. Pat. No. 3,994,986, issued Nov. 30, 1976, there is disclosed the preparation of cyclopentene from cyclopentadiene by hydrogenating cyclopentene with hydrogen gas at a ratio of 1 to 1.5 moles of hydrogen per mole of cyclopentadiene in the presence of a palladium catalyst on a carrier.
Also, in U.S. Pat. No. 3,857,894, issued Dec. 31, 1974, there is disclosed the hydrogenation of a cylopentadiene to cyclopentene in the presence of a palladium catalyst and a catalytic amount of an aqueous solution of a zinc salt having a water/zinc ratio of at least 1/1 by weight.
The cyclopentadiene employed in the formation of cyclopentene by hydrogenation is usually obtained by depolymerizing or cracking dicyclopentadiene. In order to obtain cyclopentadiene for the hydrogenation of this invention, the depolymerization of dicyclopentadiene is accomplished by heating the dimer at a temperature above 150.degree. C. under atmospheric pressure in a conventional cracking apparatus. The depolymerized material should be hydrogenated without substantial delay because it is also known that redimerization will occur upon standing.